(a) Field of the Invention
The present invention relates to a liquid crystal display having a plurality of common voltages.
(b) Description of the Related Art
Liquid crystal displays (LCDs) include two panels having pixel electrodes and a common electrode and a liquid crystal (LC) layer with dielectric anisotropy, which is interposed between the two panels. The pixel electrodes are arranged in a matrix and connected to switching elements such as thin film transistors (TFTs). The switching elements selectively transmit data voltages from data lines in response to gate signals from gate lines. The common electrode covers entire surface of one of the two panels and is supplied with a common voltage. The pixel electrode, the common electrode, and the LC layer form a LC capacitor in circuital view, which is a basic element of a pixel along with the switching element connected thereto.
In the LCD, voltages are applied to the two electrodes to generate electric field in the LC layer, and the transmittance of light passing through the LC layer is adjusted by controlling the strength of the electric field, thereby obtaining desired images. In order to prevent image deterioration due to long-time application of the unidirectional electric field, polarity of data voltages with respect to the common voltage is reversed every frame, every row, or every dot.
However, the polarity inversion causes flicker phenomenon. The flicker phenomenon is due to a kickback voltage, which is generated due to the characteristic of the switching element. That is, a pixel voltage across the LC capacitor is decreased by an amount of the kickback voltage, thereby generating the flicker phenomenon.
The kickback voltage varies depending on the position on an LCD panel. In particular, the variation of the kickback voltage is large along a row direction, i.e., the extending direction of the gate lines. It is because the difference between a gate-on voltage and a gate-off voltage, which determines the value of the kickback voltage, changes along the gate line due to the delay of the gate signals. In more detail, the kickback voltage is the largest at a position where the gate signals are first applied. However, since the drop of the gate-on voltage becomes larger as it goes away from the application point along the gate lines, the kickback voltage is decreased.
Therefore, it is suggested that a plurality of common voltages with different values should be supplied to different positions on an LCD panel to compensate the delay of the gate signals.
For example, to compensate the variation of the kickback voltage along the gate line, the common voltages having different magnitudes are applied to the left and right ends of the common electrode provided on the LCD panel.
Meanwhile, because a LC material has dielectric anisotropy, the dielectric constant of the LC material varies depending on the direction. The LC director of the LC layer in the LC capacitor is changed depending on the strength of the electric field, which in turn changes the dielectric constant of the LC layer. The change of the dielectric constant makes the capacitance of the LC capacitor be changed. Since the value of the kickback voltage depends on the capacitance of the LC capacitor, it is changed depending on the capacitance change of the LC capacitor. Generally, the variation of the kickback voltage for a data voltage applied to a pixel electrode is equal to or larger than about 17%.
However, the conventional technology applies the common voltages depending on the position on the LC panel assembly without considering the independency of the kickback voltage on the data voltages, which does not remove the flicker phenomenon.